In many energy related projects such as in the gasification of coal very large quantities of oxygen are required. In some instances, upwards 10,000 to 15,000 metric tons per day of oxygen are required. At this scale, cryogenic air distillation is the preferred method of oxygen production.
In cryogenic air distillation, air is compressed and then purified of higher boiling contaminants such as carbon dioxide, moisture and hydrocarbons. The resulting compressed and purified feed stream can be cooled within a main heat exchanger to a temperature suitable for its rectification and then introduced into a distillation column unit having a higher pressure column and a lower pressure column. The higher pressure column can be thermally linked to the lower pressure column by a condenser-reboiler that can be positioned near the base of the lower pressure column.
The feed is distilled within the higher pressure column to produce a nitrogen-rich vapor overhead and a crude liquid oxygen bottoms. The nitrogen-rich vapor overhead can be condensed within a condenser-reboiler against boiling oxygen-rich liquid collected in the base of the lower pressure column. The resulting nitrogen-rich liquid is used to reflux both the higher pressure column and the lower pressure column. The crude-liquid oxygen bottoms is introduced into the lower pressure column for further refinement. Oxygen and nitrogen product streams composed of a second nitrogen-rich vapor overhead and further oxygen-enriched liquid bottoms are extracted and can be introduced into the main heat exchanger and fully warmed in order to cool the incoming feed. In an energy related application, a liquid oxygen containing stream can be withdrawn from the lower pressure column and pumped to produce a pressurized liquid stream. The pressurized liquid stream can then be vaporized within the main heat exchanger to produce the oxygen product at pressure.
In most cryogenic rectification systems, refrigeration must be supplied in order to offset ambient heat leakage, to facilitate heat exchanger operation and to produce liquefied products. In cryogenic air distillation, the feed air is compressed in a main air compressor and then purified. Part of the air can be further compressed, partially cooled and then expanded within a turboexpander to produce a stream which can be introduced at least in part into either the higher or lower pressure columns thereby imparting refrigeration into the plant. In instances where a product fraction is desired at substantial pressure, for example an oxygen product, a further part of the feed air may be further compressed and then fully cooled and liquefied within the main heat exchanger to vaporize the pumped liquid stream. The resulting liquid stream can be expanded within a liquid expander to generate a portion of the refrigeration. In other types of plants, a nitrogen containing stream can be partially warmed and then expanded to produce refrigeration.
As plant capacity increases, a need arises to develop air separation facilities which employ multiple (often duplicate) air separation trains. This process duplication enables more cost effective construction and coldbox shipment. Each of such plants will typically employ at least one process gas turbo-expansion in order to generate the necessary refrigeration. Radial inflow turbines are typically employed in cryogenic air separation. In such turbines the diameter of such expander wheels grow in proportion to the volumetric rate of exhaust gas. This results in a costly turbo-expander (which must be purchased for each train). In addition, the turbo-expansion is often constrained to operate at modest expansion ratio and pressure. As a consequence, the thermodynamic efficiency of refrigeration is not as high as that possible given state of the art expansion ratios.
The refrigeration issues mentioned above (with respect to conventional designs) combine to measurably increase the cost to produce oxygen and nitrogen. The subject invention addresses these problems by integrating the refrigeration systems and preferably employing high efficiency refrigeration/liquefaction features in a central refrigeration source.